Description: Thallus crustose-subplacodioid, episubstratic, continuous, grey to whitish grey, up to 0.15 mm thick, usually shortly lobed at margins, sorediate/blastidiate in central parts, forming orbicular, later often confluent, up to 4(-8) cm wide patches. Center of thallus covered in abundant, grey, diffuse, granular blastidia, marginal parts with contiguous, short, 0.2-0.6(-0.8) mm wide, corticate lobes. Apothecia rather rare, subsessile, flat, 0.3-0.8 mm across, with a rust red disc, a prominent, orange, paler proper margin, and sometimes a thin, soon disappearing, often blastidiate thalline margin. Epithecium brown-orange, K+ red; hymenium and hypothecium colourless; paraphyses c. 1.5 μm thick at base, the apical cells up to 3 μm wide. Asci 8-spored, clavate, Teloschistes-type. Ascospores 1-septate, polarilocular, hyaline, ellipsoid, 13-18 x 7-10 μm, the septum often <4 μm thick (c. 1/4 of spore length). Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P- (but often K+ faintly violet-brown in section); apothecia K+ red, C+ purple. Chemistry: cortex and blastidia with the sedifolia-grey pigment; apothecia with chlorinated anthraquinones in both proper margin and disc: 7-chloroemodin (major), fragilin and emodin (minor), sometimes traces of parietin.
Note: a warm-temperate early coloniser of calciferous substrata (but very rare on pure limestone), often found on sandstone and mortar, mostly on man-made substrata (walls, monuments, roofing tiles, brick walls), common also in settlements.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)